The present invention relates to the production and application of a novel class of low-cost microcellular carbon foams and to microcellular carbon/carbon (C/C) composites manufactured therefrom.
There is a trend toward the increased use of C/C composites in space structures and satellite components because these materials possess very unique characteristics such as: high specific stiffness, high specific strength, excellent dimensional stability, near zero thermal expansion coefficients, no out-gassing and laser and radiation survivability. Sandwich structures containing such low density, high temperature core materials have many applications for, for example, high speed transport vehicles such as supersonic aircraft and outerspace structures. Conventional carbon foams demonstrate relatively low mechanical properties, such as fracture toughness, due to their amorphous morphology or low level of crystalline orientation. High cost, dictated by the need for long processing times, is also a major shortcoming of these materials.
Conventional glassy carbon foams, i.e. amorphous carbon foams, produced by the pyrolysis and graphitization of thermosetting polymer precursors such as phenolics, are very weak and can be crushed with the fingers. Such materials are therefor used primarily for nonstructural applications such as electrodes.
The carbon foams of the present invention, are preferably crystalline carbon foams produced by the pyrolysis and graphitization of thermoplastic carbon fiber precursors such as mesophase pitch or polyacrylonitrile (PAN). These foams are produced by a process wherein a blowing process aligns the anisotropic pitch molecules along the struts or boundaries of the individual abutting microcells. Such foams are mechanically very strong and can therefore be used in structural applications. They are also good conductors of electricity and heat due to their carbon nature.
Previous attempts to produce crystalline carbon foams have resulted in millimeter-sized bubbles or low levels of crystalline orientation along the struts and consequently resulted in products that demonstrated improved, but still low, fracture toughness. While such low level of crystallinity carbon foams are stronger than amorphous foams, they are brittle and can result in catastrophic failure. This behavior is clearly unacceptable for structural applications. The carbon foams of the present invention have microcellular bubbles.
Current preforms for C/C composites are generally fabricated using weaving techniques. The preforms are then densified with carbon by one of the following techniques: liquid pitch densification, chemical vapor deposition (CVD) densification, or impregnation with a high-char-yield resin. Typically, fabrication requires up to 6 months of processing time using these techniques. The product is therefore, very expensive to make. More recently, C/C composites were processed using carbon preforms infiltrated with a high-char-yield resin and carbonized. This process is repeated about three times to form a graphite preform/amorphous carbon matrix C/C composite. The product is still considerably more expensive than similar polymeric composites because of the cost of preform construction, and the repeated resin infiltration and carbonization steps while not providing entirely satisfactory mechanical properties..